[11/13] selftests/powerpc: kill off ACCESS_ONCE()

Message ID 1507573730-8083-12-git-send-email-mark.rutland@arm.com
State New
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Series
  • Preparatory work to kill off ACCESS_ONCE()
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Commit Message

Mark Rutland Oct. 9, 2017, 6:28 p.m.
For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't currently harmful.

However, for some features it is necessary to instrument reads and
writes separately, which is not possible with ACCESS_ONCE(). This
distinction is critical to correct operation.

The bulk of the kernel code can be transformed via Coccinelle to use
{READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),
and not the implementation itself. As such, it has the potential to
break homebrew ACCESS_ONCE() macros seen in some user code in the kernel
tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).

To avoid fragility if/when that transformation occurs, and to align with
the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR
selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should
be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Shuah Khan <shuah@kernel.org>
---
 tools/testing/selftests/powerpc/dscr/dscr.h              | 2 +-
 tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

-- 
1.9.1

Comments

Michael Ellerman Oct. 10, 2017, 12:45 a.m. | #1
Mark Rutland <mark.rutland@arm.com> writes:

> For several reasons, it is desirable to use {READ,WRITE}_ONCE() in

> preference to ACCESS_ONCE(), and new code is expected to use one of the

> former. So far, there's been no reason to change most existing uses of

> ACCESS_ONCE(), as these aren't currently harmful.

>

> However, for some features it is necessary to instrument reads and

> writes separately, which is not possible with ACCESS_ONCE(). This

> distinction is critical to correct operation.

>

> The bulk of the kernel code can be transformed via Coccinelle to use

> {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),

> and not the implementation itself. As such, it has the potential to

> break homebrew ACCESS_ONCE() macros seen in some user code in the kernel

> tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).

>

> To avoid fragility if/when that transformation occurs, and to align with

> the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR

> selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should

> be no functional change as a result of this patch.

>

> Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

> Cc: Michael Ellerman <mpe@ellerman.id.au>

> Cc: Paul Mackerras <paulus@samba.org>

> Cc: Shuah Khan <shuah@kernel.org>

> ---

>  tools/testing/selftests/powerpc/dscr/dscr.h              | 2 +-

>  tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +-

>  2 files changed, 2 insertions(+), 2 deletions(-)


Acked-by: Michael Ellerman <mpe@ellerman.id.au>


cheers
Mark Rutland Oct. 10, 2017, 9:32 a.m. | #2
On Tue, Oct 10, 2017 at 11:45:04AM +1100, Michael Ellerman wrote:
> Mark Rutland <mark.rutland@arm.com> writes:

> 

> > For several reasons, it is desirable to use {READ,WRITE}_ONCE() in

> > preference to ACCESS_ONCE(), and new code is expected to use one of the

> > former. So far, there's been no reason to change most existing uses of

> > ACCESS_ONCE(), as these aren't currently harmful.

> >

> > However, for some features it is necessary to instrument reads and

> > writes separately, which is not possible with ACCESS_ONCE(). This

> > distinction is critical to correct operation.

> >

> > The bulk of the kernel code can be transformed via Coccinelle to use

> > {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),

> > and not the implementation itself. As such, it has the potential to

> > break homebrew ACCESS_ONCE() macros seen in some user code in the kernel

> > tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).

> >

> > To avoid fragility if/when that transformation occurs, and to align with

> > the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR

> > selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should

> > be no functional change as a result of this patch.

> >

> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

> > Cc: Michael Ellerman <mpe@ellerman.id.au>

> > Cc: Paul Mackerras <paulus@samba.org>

> > Cc: Shuah Khan <shuah@kernel.org>

> > ---

> >  tools/testing/selftests/powerpc/dscr/dscr.h              | 2 +-

> >  tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +-

> >  2 files changed, 2 insertions(+), 2 deletions(-)

> 

> Acked-by: Michael Ellerman <mpe@ellerman.id.au>


Thanks!

Mark.

Patch

diff --git a/tools/testing/selftests/powerpc/dscr/dscr.h b/tools/testing/selftests/powerpc/dscr/dscr.h
index 18ea223b..cdb840b 100644
--- a/tools/testing/selftests/powerpc/dscr/dscr.h
+++ b/tools/testing/selftests/powerpc/dscr/dscr.h
@@ -39,7 +39,7 @@ 
 #define rmb()  asm volatile("lwsync":::"memory")
 #define wmb()  asm volatile("lwsync":::"memory")
 
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
 
 /* Prilvilege state DSCR access */
 inline unsigned long get_dscr(void)
diff --git a/tools/testing/selftests/powerpc/dscr/dscr_default_test.c b/tools/testing/selftests/powerpc/dscr/dscr_default_test.c
index df17c3b..9e1a37e 100644
--- a/tools/testing/selftests/powerpc/dscr/dscr_default_test.c
+++ b/tools/testing/selftests/powerpc/dscr/dscr_default_test.c
@@ -27,7 +27,7 @@  static void *do_test(void *in)
 		unsigned long d, cur_dscr, cur_dscr_usr;
 		unsigned long s1, s2;
 
-		s1 = ACCESS_ONCE(sequence);
+		s1 = READ_ONCE(sequence);
 		if (s1 & 1)
 			continue;
 		rmb();